Disk drive calibrating a velocity profile prior to writing a spiral track

ABSTRACT

A disk drive is disclosed comprising a head actuated over a disk, and a ramp near an outer diameter of the disk, wherein the ramp for unloading the head off the disk. The head is positioned at an inner diameter of the disk and then moved toward the ramp in response to a velocity profile. An interval is measured from a beginning of the movement until the head begins unloading onto the ramp, and the velocity profile is adjusted in response to the measured interval.

BACKGROUND

When manufacturing a disk drive, concentric servo sectors 2 ₀-2 _(N) arewritten to a disk 4 which define a plurality of radially-spaced,concentric servo tracks 6 as shown in the prior art disk format ofFIG. 1. A plurality of concentric data tracks are defined relative tothe servo tracks 6, wherein the data tracks may have the same or adifferent radial density (tracks per inch (TPI)) than the servo tracks6. Each servo sector (e.g., servo sector 2 ₄) comprises a preamble 8 forsynchronizing gain control and timing recovery, a sync mark 10 forsynchronizing to a data field 12 comprising coarse head positioninginformation such as a track number, and servo bursts 14 which providefine head positioning information. The coarse head position informationis processed to position a head over a target data track during a seekoperation, and the servo bursts 14 are processed to maintain the headover a centerline of the target data track while writing or reading dataduring a tracking operation.

In the past, external servo writers have been used to write theconcentric servo sectors 2 ₀-2 _(N) to the disk surface duringmanufacturing. External servo writers employ extremely accurate headpositioning mechanics, such as a laser interferometer, to ensure theconcentric servo sectors 2 ₀-2 _(N) are written at the proper radiallocation from the outer diameter of the disk to the inner diameter ofthe disk. However, external servo writers are expensive and require aclean room environment so that a head positioning pin can be insertedinto the head disk assembly (HDA) without contaminating the disk. Thus,external servo writers have become an expensive bottleneck in the diskdrive manufacturing process.

The prior art has suggested various “self-servo” writing methods whereinthe internal electronics of the disk drive are used to write theconcentric servo sectors independent of an external servo writer. Forexample, U.S. Pat. No. 5,668,679 teaches a disk drive which performs aself-servo writing operation by writing a plurality of spiral servotracks to the disk which are then processed to write the concentricservo sectors along a circular path. Each spiral servo track is writtento the disk as a high frequency signal (with missing bits), wherein theposition error signal (PES) for tracking is generated relative to timeshifts in the detected location of the spiral servo tracks. The readsignal is rectified and low pass filtered to generate a triangularenvelope signal representing a spiral servo track crossing, wherein thelocation of the spiral servo track is detected by detecting a peak inthe triangular envelope signal relative to a clock synchronized to therotation of the disk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of radiallyspaced, concentric tracks defined by a plurality of product servosectors.

FIG. 2A illustrates an embodiment of the present invention wherein aplurality of spiral tracks are written to the disk for use in writingproduct servo sectors to the disk.

FIG. 2B illustrates an embodiment of the present invention wherein eachspiral track is written over multiple revolutions of the disk.

FIG. 3A shows an embodiment of the present invention wherein a disklocked clock is synchronized by clocking a modulo-N counter relative towhen the sync marks in the spiral tracks are detected.

FIG. 3B shows an eye pattern generated by reading the spiral track,including the sync marks in the spiral track.

FIG. 4 illustrates writing of product servo sectors using the disklocked clock to servo on the spiral tracks.

FIGS. 5A-5C show an embodiment of the present invention wherein a syncmark seam is written in a bootstrap spiral track in response to aspindle BEMF voltage.

FIG. 6 is a flow diagram according to an embodiment of the presentinvention wherein the sync mark seam is written by switching sync marksin response to the spindle BEMF voltage.

FIG. 7 is a flow diagram according to an embodiment of the presentinvention wherein a disk locked clock is synchronized to zero crossingsin the spindle BEMF voltage and then used to write the sync mark seam inthe bootstrap spiral track.

FIG. 8 shows a velocity profile according to an embodiment of thepresent invention wherein the bootstrap spiral track is written with asubstantially constant velocity until the head contacts a ramp at anouter edge of the disk.

FIGS. 9A and 9B show an embodiment of the present invention wherein thedisk locked clock is synchronized to the bootstrap spindle BEMF voltage,and the modulo-N counter initialized in response to detecting the syncmark seam in the bootstrap spiral track.

FIG. 10A is a flow diagram according to an embodiment of the presentinvention for initializing the modulo-N counter in response to detectingthe sync mark seam in the bootstrap spiral track.

FIG. 10B is a flow diagram according to an embodiment of the presentinvention wherein the modulo-N counter is initialized by shifting itincrementally (e.g., one spindle BEMF zero crossing) in order tomaintain stability of the servo loop.

FIG. 11A shows an embodiment of the present invention wherein the syncmark seam in the bootstrap spiral track is detected in response to afirst and second weighted measure of the read signal corresponding tothe first and second sync marks.

FIG. 11B is a flow diagram according to an embodiment of the presentinvention wherein the sync mark seam in the bootstrap spiral is detectedwhen the first and second weighted measures switch amplitude.

FIG. 12 is a flow diagram according to an embodiment of the presentinvention for calibrating a velocity profile used to write the bootstrapspiral track.

FIG. 13 is a flow diagram according to an embodiment of the presentinvention wherein when calibrating the velocity profile a voice coilmotor (VCM) BEMF voltage is used to seek the head over the disk untilthe head contacts the ramp.

FIG. 14A illustrates an embodiment of the present invention wherein thedisk locked clock is used to estimate a radial location of the headrelative to a rotational angle of the disk while reading the bootstrapspiral track.

FIG. 14B is a flow diagram according to an embodiment of the presentinvention wherein ID and OD circular reference tracks are written on thedisk by servoing on the bootstrap spiral track.

FIG. 15A shows an embodiment of the present invention wherein a gappedspiral track is written while moving the head in a radial directionopposite of that used to write the bootstrap spiral track.

FIG. 15B is a flow diagram according to an embodiment of the presentinvention wherein the velocity profile is adjusted in response toreading the ID and OD circular reference tracks as well as the bootstrapspiral track.

FIGS. 16A and 16B show an embodiment of the present invention whereinthe bootstrap spiral track is written at a first slope, and the gappedspiral tracks are written at a second, different slope.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2A shows a disk 18 comprising a plurality of spiral tracks 20 ₀-20_(N), wherein each spiral track 20 comprises a high frequency signal 22interrupted by sync marks 24 (FIG. 3B). Each spiral track 20 in FIG. 2Aspans a fractional disk revolution; however, in other embodiments eachspiral track 20 may span multiple disk revolutions as shown in FIG. 2B.As described below, in one embodiment the spiral tracks 20 ₀-20 _(N) areused to servo the head over the disk 18 while writing product servosectors that define concentric servo tracks such as shown in FIG. 1.Conventionally, the spiral tracks 20 ₀-20 _(N) were written using anexternal spiral track writer or a media writer, and then the controlcircuitry internal to the disk drive used to process the spiral tracks20 ₀-20 _(N) in order to self servo write the product servo sectors thatdefine the concentric servo tracks. In some embodiments of the presentinvention, the spiral tracks 20 ₀-20 _(N) may be written by the controlcircuitry internal to each disk drive thereby obviating the bottleneckand expense of an external spiral track writer or media writer.

FIG. 3B illustrates an “eye” pattern in the read signal generated whenthe head crosses over a spiral track 20. The read signal representingthe spiral track crossing comprises high frequency transitions 22interrupted by sync marks 24 at a predetermined interval. When the headmoves in the radial direction, the eye pattern will shift (left orright) while the sync marks 24 remain fixed (ideally). The shift in theeye pattern (detected from the high frequency signal 22) relative to thesync marks 24 provides the off-track information (spiral position errorsignal (PES)) for servoing the head.

FIG. 3A shows an embodiment of the present invention wherein a saw-toothwaveform 26 is generated by clocking a modulo-N counter with a disklocked clock, wherein the frequency of the disk locked clock is adjusteduntil the sync marks 24 in the spiral tracks 20 ₀-20 _(N) are detectedat a target modulo-N count value. The disk locked clock may be generatedusing any suitable circuitry, such as a phase locked loop (PLL). As eachsync mark 24 in the spiral tracks 20 ₀-20 _(N) is detected, the value ofthe modulo-N counter represents the phase error for adjusting the PLL.

The sync marks 24 in the spiral tracks 20 ₀-20 _(N) may comprise anysuitable pattern, and in one embodiment, a pattern that is substantiallyshorter than the sync mark 10 in the conventional product servo sectors2 of FIG. 1. A shorter sync mark 24 allows the spiral tracks 20 ₀-20_(N) to be written to the disk 18 using a steeper slope (by moving thehead faster radially over the disk 18) which reduces the time requiredto write each spiral track 20 ₀-20 _(N). In an embodiment describedbelow, a bootstrap spiral track may be written to the disk usingdifferent sync marks, wherein the change in sync marks defines a syncmark seam within the bootstrap spiral track.

In one embodiment, the disk locked clock is further synchronized bygenerating a timing recovery measurement from the high frequency signal22 between the sync marks 24 in the spiral tracks 20 ₀-20 _(N).Synchronizing the disk locked clock to the high frequency signal 22helps maintain proper radial alignment (phase coherency) of the Graycoded track addresses in the product servo sectors. The timing recoverymeasurement may be generated in any suitable manner. In one embodiment,the disk locked clock is used to sample the high frequency signal 22 andthe signal sample values are processed to generate the timing recoverymeasurement. The timing recovery measurement adjusts the phase of thedisk locked clock (PLL) so that the high frequency signal 22 is sampledsynchronously. In this manner, the sync marks 24 provide a coarse timingrecovery measurement and the high frequency signal 22 provides a finetiming recovery measurement for maintaining synchronization of the disklocked clock.

FIG. 4 illustrates an embodiment of the present invention for writingthe product servo sectors 28 ₀-28 _(N) to the disk 18 aftersynchronizing the disk locked clock in response to at least the syncmarks 24 in the spiral tracks 20 ₀-20 _(N). In the embodiment of FIG. 4,the dashed lines represent the centerlines of the servo tracks. The syncmarks in the spiral tracks 20 ₀-20 _(N) are written so that there is ashift of two sync marks 24 in the eye pattern (FIG. 3B) between servotracks. In an alternative embodiment, the sync marks 24 in the spiraltracks 20 ₀-20 _(N) are written so that there is a shift of N sync marksin the eye pattern between servo tracks. In the embodiment of FIG. 4,each spiral track 20 ₀-20 _(N) is wider than a servo track, however, inan alternative embodiment the width of each spiral track 20 ₀-20 _(N) isless than or proximate the width of a servo track.

The spiral PES for maintaining the head along a servo track (tracking)while writing the product servo sectors 28 ₀-28 _(N) may be generatedfrom the spiral tracks 20 ₀-20 _(N) in any suitable manner. In oneembodiment, the PES is generated by detecting the eye pattern in FIG. 3Busing an envelope detector and detecting a shift in the enveloperelative to the sync marks 24. In one embodiment, the envelope isdetected by integrating the high frequency signal 22 and detecting ashift in the resulting ramp signal. In an alternative embodiment, thehigh frequency signal 22 between the sync marks 24 in the spiral tracksare demodulated as servo bursts and the PES generated by comparing theservo bursts in a similar manner as the servo bursts 14 in the productservo sectors (FIG. 1).

Once the head is tracking on a servo track, the product servo sectors 28₀-28 _(N) are written to the disk using the disk locked clock. Writecircuitry is enabled when the modulo-N counter reaches a predeterminedvalue, wherein the disk locked clock clocks the write circuitry to writethe product servo sector 28 to the disk. The spiral tracks 20 ₀-20 _(N)on the disk are processed in an interleaved manner to account for theproduct servo sectors 28 ₀-28 _(N) overwriting a spiral track. Forexample, when writing the product servo sectors 28 ₁ to the disk, spiraltrack 20 ₁ is processed initially to generate the spiral PES trackingerror and the disk locked clock timing recovery measurement. When theproduct servo sectors 28 ₁ begin to overwrite spiral track 20 ₁, spiraltrack 20 _(o) is processed to generate the spiral PES tracking error andthe disk locked clock timing recovery measurement.

FIG. 5A shows a disk drive according to an embodiment of the presentinvention comprising a head 30 actuated over a disk 18, a spindle motor32 operable to rotate the disk 18, and control circuitry 34 operable toexecute the flow diagram of FIG. 6. A bootstrap spiral track 36 iswritten to the disk 18 (FIG. 5B), wherein the bootstrap spiral track 36comprises a high frequency signal interrupted by sync marks (FIG. 3B).While writing the bootstrap spiral track 36, a spindle backelectromotive force (BEMF) voltage generated by the spindle motor 32 ismeasured (step 38), a first sync mark is written while writing a firstpart of the bootstrap spiral track (step 40), and a second sync mark iswritten while writing a second part of the bootstrap spiral track (step44). A switch between writing the first sync mark and the second syncmark is made (step 42) in response to a spindle BEMF voltage, therebygenerating a sync mark seam 46 in the bootstrap spiral track.

FIG. 5C shows an embodiment of the present invention wherein the controlcircuitry 34 is operable to switch between writing the first sync markand the second sync mark in response to zero crossings in the spindleBEMF voltage. In the example of FIG. 5C, the sync mark seam 46 iswritten in the bootstrap spiral track 36 at a target interval (e.g.,halfway) between two consecutive zero crossings in the spindle BEMFvoltage. As described below, in one embodiment the sync mark seam 46 isused to synchronize to the bootstrap spiral track 36 by initializing themodulo-N counter 26 based on the spindle BEMF voltage zero crossing thatoccurs after detecting the sync mark seam 46.

FIG. 7 is a flow diagram according to an embodiment of the presentinvention which extends on the flow diagram of FIG. 6, wherein the headis positioned at the inner diameter of the disk (step 48), for example,by pressing an actuator arm 35 (FIG. 5B) against an inner diameter crashstop (not shown). The spindle BEMF voltage is measured (step 38) and thedisk locked clock is synchronized to the zero crossings in the spindleBEMF voltage (step 50). When the disk locked clock reaches a first value(step 52), the control circuitry launches the head toward a middlediameter of the disk (step 54). The bootstrap spiral track is writtenusing the first sync mark (step 40) until the disk locked clock reachesa second value corresponding to the head reaching a target radiallocation (step 42). The bootstrap spiral track is then written using thesecond sync mark (44), thereby defining the sync mark seam 46 in thebootstrap spiral track.

In one embodiment, the control circuitry accelerates the head toward themiddle diameter of the disk while writing the first part of thebootstrap spiral track as illustrated by the velocity profile shown inFIG. 8. Proximate to switching between writing the first sync mark andthe second sync mark, the control circuitry moves the head toward theouter diameter of the disk at a substantially constant velocity. Thatis, the sync mark seam 46 is written in the bootstrap spiral trackproximate to the transition between an acceleration segment and aconstant velocity segment as illustrated in FIG. 8.

In one embodiment, after launching the head toward the middle diameterof the disk when writing the bootstrap spiral track, the disk willrotate through a known angle before writing the sync mark seam 46,wherein the known angle corresponds to a number of cycles of the disklocked clock. Accordingly, in one embodiment the head is launched whenthe disk locked clock reaches a first value that is computed relative toa number of cycles before writing the sync mark seam:

${first\_ value} = {{{countsPerRe}\;\nu} - {\frac{1}{2}\frac{{countsPerRe}\;\nu}{{spindleZXPerRe}\;\nu}} - {second\_ value}}$where countsPerRev represents the total number of cycles (counts) of thedisk locked clock over a full revolution of the disk, spindleZXPerRevrepresents the number of spindle BEMF zero crossings per revolution ofthe disk, and the second value represents the disk rotation angle fromthe launch point until the sync mark seam is written. Launching the headwhen the disk locked clock reaches the first value computed from theabove equation will cause the sync mark seam 46 to be written at aninterval that is halfway between two consecutive spindle BEMF zerocrossings as illustrated in FIG. 5C.

In one embodiment, the sync mark seam 46 in the bootstrap spiral trackis used to resynchronize the disk locked clock, for example, after apower cycle. Synchronizing the disk locked clock to the sync mark seam46 essentially initializes the radial and circumferential location ofthe head to a known state. In addition, after synchronizing the disklocked clock to the sync mark seam 46 the head may be accurately servoedradially over the disk based on the bootstrap spiral track relative tothe rotational angle of the disk as determined by the disk locked clock.

FIGS. 9A and 9B illustrate an embodiment of the present invention forresynchronizing the disk locked clock to the sync mark seam 46 as isunderstood with reference to the flow diagram of FIG. 10A. The head ispositioned over the disk (step 56), for example, by loading the head offa ramp 57 (FIG. 5B). The spindle BEMF voltage is measured (step 58) andthe disk locked clock synchronized to the zero crossings in the spindleBEMF voltage (step 60). The disk locked clock is used to clock themodulo-N counter 26 (step 62) such that the counter wraps at anarbitrary zero crossing 63 (FIG. 9A). The head is then moved radiallyover the disk in order to search for the sync mark seam (step 64). Whenthe sync mark seam is found (step 66), the modulo-N counter isinitialized (step 68). For example, in one embodiment the modulo-Ncounter is initialized so it wraps at the spindle BEMF zero crossing 65that follows the detection of the sync mark seam as illustrated in FIG.9B.

FIG. 10B shows a flow diagram according to an embodiment of the presentinvention which extends on the flow diagram of FIG. 10A, wherein themodulo-N counter is adjusted incrementally by a spindle BEMF zerocrossing so as to maintain stability in the servo loop. After detectingthe sync mark seam (step 66), if the modulo-N counter is not alignedcorrectly with the following spindle BEMF zero crossing (step 70), themodulo-N counter is shifted by an amount corresponding to a singlespindle BEMF zero crossing (step 72). The shift in the modulo-N counterinduces a transient in the servo loop similar to a radially shift of thehead. Accordingly, the servo system is allowed to settle to account forthe transient (step 74). The process is then repeated starting at step70 until the modulo-N counter wraps at the target spindle BEMF zerocrossing. In the example of FIG. 9B, the process is repeated twice inorder to shift the modulo-N counter by two spindle BEMF zero crossingsso that the modulo-N counter wraps at zero crossing 65.

Any suitable technique may be employed to detect the sync mark seam 46in the embodiments of the present invention. For example, the sync markseam 46 may be detected using first and second correlators matched tothe first and second sync marks. The sync mark seam 46 may be detectedwhen there is a switch between the output of the correlators. However,noise in the read signal may reduce the accuracy of the correlatorsleading to a false detection of the sync mark seam 46.

FIGS. 11A and 11B show an embodiment of the present invention fordetecting a sync mark seam in a spiral track more reliably. Whensearching for the sync mark seam 46 of a spiral track, the controlcircuitry moves the head radially over the disk (step 76) and reads thespiral track to generate a read signal (step 78). The read signalrepresenting the high frequency signal 22 (FIG. 3B) is integrated (step80), and when the first sync mark is detected (step 82), the integratedread signal is accumulated into a first weighted measure (step 84). Whenthe second sync mark is detected (step 82), the integrated read signalis accumulated into a second weighted measure (step 86). The sync markseam in the spiral track is detected in response to the first and secondweighted measures (step 88). In one embodiment, the sync mark seam isdetected proximate to when an amplitude of the second weighted measurecrosses an amplitude of the first weighted measure as illustrated inFIG. 11A. Weighting the sync mark detection relative to the amplitude ofthe read signal improves the accuracy in detecting the sync mark seam byreducing the significance of the sync marks near the edge of the spiraltrack crossing where the signal-to-noise ratio is less (FIG. 3B).

In one embodiment, the control circuitry calibrates a velocity profileprior to writing the bootstrap spiral track 36 to the disk (FIG. 5B),wherein the velocity profile (e.g., FIG. 8) corresponds to a desiredslope for the spiral track across the radius of the disk. FIG. 12 is aflow diagram illustrating an embodiment for calibrating the velocityprofile wherein the head is positioned at an inner diameter of the disk(step 90) and then moved toward the ramp in response to the velocityprofile (step 92). An interval is measured (step 94) from a beginning ofthe movement until the head begins unloading onto the ramp 57 of FIG. 5B(step 96). The velocity profile is then adjusted in response to themeasured interval (step 98). For example, the velocity profile isadjusted based on the difference between the measured interval and atarget interval.

In one embodiment, the interval at step 94 is measured relative to thedisk locked clock which represents the rotational phase of the disk. Inthis embodiment, the target interval corresponds to a target rotationalphase of the disk (which may be less or more than one revolution). Inone embodiment, the velocity profile is adjusted and the flow diagram ofFIG. 12 repeated until the measured interval substantially matches thetarget interval.

Any suitable velocity profile may be employed in the embodiments of thepresent invention. FIG. 8 shows an example velocity profile comprisingan acceleration segment for accelerating the head away from the innerdiameter of the disk, and a constant velocity segment for moving thehead at a substantially constant velocity until the head beginsunloading onto the ramp. In this embodiment, the constant velocity ofthe head as it contacts the ramp is consistent with the constantvelocity employed when unloading the head onto the ramp during a normalunload operation.

FIG. 13 is a flow diagram according to an embodiment of the presentinvention which expands on the flow diagram of FIG. 12, wherein the diskdrive in this embodiment comprises a voice coil motor (VCM) 33 forrotating an actuator arm 35 about a pivot (FIG. 5A) in order to move thehead radially over the disk. After positioning the head at the innerdiameter of the disk (step 90), for example by pressing the actuator arm35 against an inner diameter crash stop, parameters of the VCM aremeasured (step 100), such as the resistance and inductance of the voicecoil. The spindle motor BEMF voltage is measured (step 102) and the disklocked clock is synchronized to the zero crossings of the spindle BEMFvoltage (step 104). The head is moved away from the inner diameter ofthe disk (step 105) and the VCM BEMF voltage is measured (step 106). TheVCM BEMF voltage is evaluated to estimate a velocity of the head (aftercompensating for the effect of the parameters measured at step 100). Theestimated velocity of the head (and optionally other states) areevaluated relative to the velocity profile in order to adjust thevelocity of the VCM (step 108). The process is repeated from step 106until the head contacts the ramp (step 96). If the intervening interval(measured at step 94) does not match a target interval (step 110), thevelocity profile is adjusted (step 98) and the flow diagram repeatedstarting at step 90 until the measured interval substantially matchesthe target interval (step 110).

After calibrating the velocity profile, the bootstrap spiral track 36 iswritten to the disk using the calibrated velocity profile. In anembodiment shown in FIG. 8, when writing the bootstrap spiral track 36the head is accelerated away from the inner diameter of the disk andthen moved at a substantially constant velocity until the head contactsthe ramp 57. After the head contacts the ramp, the head is decelerateduntil it reaches the parked position on the ramp 57. In this manner, theresulting bootstrap spiral track 36 comprises an increasing slope whilethe head accelerates away from the inner diameter of the disk and asubstantially constant slope until the head contacts the ramp asillustrated in FIG. 16A. In one embodiment, the substantially constantslope of the bootstrap spiral track 36 up to the ramp 57 helps maximizethe usable area of the disk for writing the product servo sectors byenabling an outer diameter circular reference track to be written verynear the outer edge of the disk as described below.

FIG. 14A illustrates an embodiment of the present invention whereinafter writing the bootstrap spiral track, the control circuitry servoson the bootstrap spiral track to write at least one circular referencetrack (e.g., 112A) on the disk 18. A velocity profile is adjusted inresponse to the circular reference track 112A, and a plurality of spiraltracks are written to the disk in response to the velocity profile. Inone embodiment, the velocity profile for writing the spiral tracks usingthe circular reference track is similar to the velocity profile shown inFIG. 8, except the head is decelerated before reaching the ramp. In oneembodiment, the control circuitry seeks the head radially over the diskback and forth in order to write the spiral tracks, wherein the velocityprofile is adjusted each time the head crosses over the circularreference track.

FIG. 14B is a flow diagram according to the embodiment of the presentinvention which is understood with reference to FIG. 14A. The controlcircuitry positions the head at an inner diameter of the disk (step114), such as by pressing the actuator arm against the inner diametercrash stop. The disk locked clock is then synchronized to the bootstrapspiral track (step 116), wherein in one embodiment the disk locked clockis first synchronized to the zero crossings of the spindle BEMF voltageas described above. The head is then moved radially while searching forthe sync mark seam (step 118), and when the sync mark seam is located,the modulo-N counter is initialized as described above.

Once the sync mark seam has been located and the modulo-N counter isinitialized, the radial location of the head is known relative to thespiral bootstrap track. At this point, the control circuitry is able toservo the head to any desired radial location by servoing on thebootstrap spiral track relative to the modulo-N counter as describedabove. In one embodiment, the control circuitry positions the head nearthe inner diameter of the disk and writes a circular reference track112A (FIG. 14A) while servoing on the bootstrap spiral track (step 120).In one embodiment, the inner diameter circular reference track 112A iswritten proximate the sync mark seam 46, which in one embodimentcorresponds to the end of the constant velocity segment of the velocityprofile used to write the spiral tracks. Accordingly, in this embodimentafter writing one of the spiral tracks the inner diameter referencetrack 112A is read in order to adjust the velocity profile.

Referring again to FIG. 14B, the control circuitry seeks the head to anouter diameter location proximate the ramp (step 122) in order to writean outer diameter circular reference track 112B as shown in FIG. 14A.The control circuitry estimates the radial location of the ramp 57 inFIG. 14A by evaluating the rotational phase of the disk (as determinedfrom the disk locked clock) relative to the spiral bootstrap spiraltrack 36. That is, when writing the bootstrap spiral track 36 to thedisk, the rotational phase of the disk (disk locked clock) is saved whenthe head contacts the ramp 57. The control circuitry then positions thehead at the outer diameter of the disk by servoing the head on thebootstrap spiral track until the rotational phase θ of the diskcorresponds to the location of the ramp (minus a back-off delta) asillustrated in FIG. 14A, and then writes the outer diameter circularreference track 112B (step 124). The inner diameter and outer diametercircular reference tracks 112A and 112B are then used to adjust thevelocity profile while writing a plurality of spiral tracks to the disk(step 126).

Similar to the inner diameter reference track 112A, in one embodimentthe outer diameter reference track 112B is written at the edge of theconstant velocity segment of the velocity profile used to write thespiral tracks. As the head crosses over and reads the circular referencetracks at both the inner and outer diameter, the velocity profile isadjusted. For example as described above, the intervening intervalbetween the reference tracks during each seek is measured, compared to atarget interval, and the difference used to adjust the velocity profile.

FIG. 15A shows an embodiment of the present invention for writing theplurality of spiral tracks while adjusting the velocity profile inresponse to the circular reference tracks. In this embodiment, thebootstrap spiral track 36 is also used to adjust the velocity profile bymeasuring an interval between spiral track crossings, comparing theinterval to a target interval, and adjusting the velocity profile inresponse to the difference. Accordingly in this embodiment, the writingof each spiral track is paused in order to read the bootstrap spiraltrack as the head approaches an expected spiral track crossing, andtherefore the spiral tracks are written with gaps to allow the head toread the bootstrap spiral track.

Referring again to FIG. 15A, the control circuitry positions the head ata first radial location (inner diameter in this example), and moves thehead from the first radial location toward a second radial location(outer diameter) while writing the bootstrap spiral track 36 to the disk18. The head is then positioned near the second radial location (outerdiameter), and moved from the second radial location toward the firstradial location (inner diameter) while writing a gapped spiral track 114to the disk 18. Accordingly in this embodiment, the gapped spiral track114 is written in the opposite radial direction that the bootstrapspiral track 36 is written. Writing the gapped spiral track 114 in theopposite direction increases the number of bootstrap spiral crossingsand therefore the number of samples used to adjust the velocity profile.

FIG. 15B is a flow diagram according to a more detailed embodiment ofthe present invention, wherein the control circuitry seeks the head tothe outer diameter of the disk using the velocity profile (step 128),and then begins a new seek operation in the opposite direction towardthe outer diameter circular reference track (step 130). When the outerdiameter circular reference track is detected (step 132) a velocityprofile error is measured (step 134). The control circuitry then beginswriting a gapped spiral track to the disk (step 136) while seeking thehead radially over the disk at the substantially constant velocity ofthe velocity profile. When the head approaches the bootstrap spiraltrack 36 as shown in FIG. 15A (step 138), the writing of the gappedspiral track is paused (step 140) in order to read the bootstrap spiraltrack and measure a corresponding velocity profile error (step 142). Thegapped spiral track continues to be written across the radius of thedisk (step 144) while periodically pausing to read the bootstrap spiraltrack and generate another velocity profile error. When the head reachesthe inner diameter circular reference track (step 146), the controlcircuitry stops writing the gapped spiral track (step 148), and readsthe inner diameter reference track in order to measure a velocityprofile error (step 150). The control circuitry then adjusts thevelocity profile using the measured velocity profile errors (step 152)prior to repeating the flow diagram (starting at step 128) in order towrite another gapped spiral track.

In one embodiment, the control circuitry writes the bootstrap spiraltrack 36 having a first substantially constant slope, and writes thegapped spiral tracks having a second substantially constant slopedifferent than the first substantially constant slope. This isillustrated in FIG. 16A wherein the slope of the gapped spiral tracks isgreater than the slope of the bootstrap spiral track, or in analternative embodiment shown in FIG. 16B, the slope of the gapped spiraltracks may be less than the slope of the bootstrap spiral track. FIGS.16A and 16B also illustrate an embodiment wherein the bootstrap spiraltrack is written with a substantially constant slope until the headcontacts the ramp, whereas the gapped spiral tracks are written with asubstantially constant slope until the head reaches the inner diametercircular reference track. Also in the embodiment of FIGS. 16A and 16Bthe gapped spiral tracks begin when the head reaches the constantvelocity segment of the velocity profile (e.g., when the head reachesthe outer diameter circular reference track).

In one embodiment, after writing one of the gapped spiral tracks fromthe outer diameter to the inner diameter, the control circuitry seeksthe head back to the outer diameter of the disk (step 128 of FIG. 15B)using the same velocity profile used to write the gapped spiral tracks.Since the velocity profile for writing the gapped spiral tracks isdifferent than that used to write the bootstrap spiral track, the headwill cross the bootstrap spiral track while seeking back to the outerdiameter of the disk. In one embodiment, each time the head crosses thebootstrap spiral track while seeking back to the outer diameter of thedisk, a velocity profile error is measured and used to adjust thevelocity profile for writing the gapped spiral tracks.

Any suitable control circuitry may be employed to implement the flowdiagrams in the embodiments of the present invention, such as anysuitable integrated circuit or circuits. For example, the controlcircuitry may be implemented within a read channel integrated circuit,or in a component separate from the read channel, such as a diskcontroller, or certain steps described above may be performed by a readchannel and others by a disk controller. In one embodiment, the readchannel and disk controller are implemented as separate integratedcircuits, and in an alternative embodiment they are fabricated into asingle integrated circuit or system on a chip (SOC). In addition, thecontrol circuitry may include a suitable preamp circuit implemented as aseparate integrated circuit, integrated into the read channel or diskcontroller circuit, or integrated into an SOC.

In one embodiment, the control circuitry comprises a microprocessorexecuting instructions, the instructions being operable to cause themicroprocessor to perform the steps of the flow diagrams describedherein. The instructions may be stored in any computer-readable medium.In one embodiment, they may be stored on a non-volatile semiconductormemory external to the microprocessor, or integrated with themicroprocessor in a SOC. In another embodiment, the instructions arestored on the disk and read into a volatile semiconductor memory whenthe disk drive is powered on. In yet another embodiment, the controlcircuitry comprises suitable logic circuitry, such as state machinecircuitry.

What is claimed is:
 1. A disk drive comprising: a disk; a head actuatedover the disk; a ramp near an outer diameter of the disk, wherein theramp for unloading the head off the disk; and control circuitry operableto: position the head at an inner diameter of the disk; move the headtoward the ramp in response to a velocity profile; measure an intervalfrom a beginning of the movement until the head begins unloading ontothe ramp; adjust the velocity profile in response to the measuredinterval; synchronize a disk locked clock to a rotation of the disk; andmeasure the interval relative to the disk locked clock.
 2. The diskdrive as recited in claim 1, wherein the control circuitry is furtheroperable to write a spiral track to the disk in response to the adjustedvelocity profile.
 3. The disk drive as recited in claim 1, wherein thevelocity profile comprises: an acceleration segment for accelerating thehead away from the inner diameter of the disk; and a constant velocitysegment for moving the head at a substantially constant velocity untilthe head begins unloading onto the ramp.
 4. The disk drive as recited inclaim 1, further comprising a voice coil motor (VCM) for actuating thehead over the disk, wherein the control circuitry is further operableto: measure a VCM back electromotive force (BEMF) voltage generated bythe VCM; and move the head toward the ramp in response to the velocityprofile and the VCM BEMF voltage.
 5. The disk drive as recited in claim1, further comprising a spindle motor for rotating the disk, wherein thecontrol circuitry is further operable to: measure a spindle backelectromotive force (BEMF) voltage generated by the spindle motor; andsynchronize the disk locked clock in response to the spindle BEMFvoltage.
 6. The disk drive as recited in claim 1, wherein the controlcircuitry is further operable to repeatedly move the head from the innerdiameter of the disk to the ramp and adjust the velocity profile untilthe measured interval substantially matches a target interval.
 7. A diskdrive comprising: a disk; a head actuated over the disk; a ramp near anouter diameter of the disk, wherein the ramp for unloading the head offthe disk; and control circuitry operable to: position the head at aninner diameter of the disk; write a spiral track while moving the headtoward the ramp at a substantially constant velocity until the headcontacts the ramp; and after the head contacts the ramp, decelerate thehead.
 8. The disk drive as recited in claim 7, wherein the controlcircuitry is further operable to accelerate the head away from the innerdiameter of the disk while writing the spiral track.
 9. The disk driveas recited in claim 8, wherein the spiral track comprises an increasingslope while the head accelerates away from the inner diameter of thedisk and a substantially constant slope until the head contacts theramp.
 10. A method of operating a disk drive, the disk drive comprisinga head actuated over a disk, and a ramp near an outer diameter of thedisk, wherein the ramp for unloading the head off the disk, the methodcomprising: positioning the head at an inner diameter of the disk;moving the head toward the ramp in response to a velocity profile;measuring an interval from a beginning of the movement until the headbegins unloading onto the ramp; adjusting the velocity profile inresponse to the measured interval; synchronizing a disk locked clock toa rotation of the disk; and measuring the interval relative to the disklocked clock.
 11. The method as recited in claim 10, further comprisingwriting a spiral track to the disk in response to the adjusted velocityprofile.
 12. The method as recited in claim 10, wherein the velocityprofile comprises: an acceleration segment for accelerating the headaway from the inner diameter of the disk; and a constant velocitysegment for moving the head at a substantially constant velocity untilthe head begins unloading onto the ramp.
 13. The method as recited inclaim 10, wherein the disk drive further comprises a voice coil motor(VCM) for actuating the head over the disk, the method furthercomprising: measuring a VCM back electromotive force (BEMF) voltagegenerated by the VCM; and moving the head toward the ramp in response tothe velocity profile and the VCM BEMF voltage.
 14. The method as recitedin claim 10, wherein the disk drive further comprises a spindle motorfor rotating the disk, the method further comprising: measuring aspindle back electromotive force (BEMF) voltage generated by the spindlemotor; and synchronizing the disk locked clock in response to thespindle BEMF voltage.
 15. The method as recited in claim 10, furthercomprising repeatedly moving the head from the inner diameter of thedisk to the ramp and adjusting the velocity profile until the measuredinterval substantially matches a target interval.
 16. A method ofoperating a disk drive, the disk drive comprising a head actuated over adisk, and a ramp near an outer diameter of the disk, wherein the rampfor unloading the head off the disk, the method comprising: positioningthe head at an inner diameter of the disk; writing a spiral track whilemoving the head toward the ramp at a substantially constant velocityuntil the head contacts the ramp; and after the head contacts the ramp,decelerating the head.
 17. The method as recited in claim 16, furthercomprising accelerating the head away from the inner diameter of thedisk while writing the spiral track.
 18. The method as recited in claim17, wherein the spiral track comprises an increasing slope while thehead accelerates away from the inner diameter of the disk and asubstantially constant slope until the head contacts the ramp.